Chapter 6: Active Sites for NO_x Uptake, and the Nature of Stored NO_x Species

The storage on and release of NO_x from a NO_x storage material (usually BaO) during lean- and rich-engine operation are primary attributes of the NSR catalyst system, with these processes involving the interconversion of Ba-oxide with Ba(NO₃)₂. Significant morphological changes in the Ba-based storage material take place during these processes due, in part, to considerable differences in the sizes of their oxide and nitrate phases. Characterizing and understanding these morphological phases has been essential for the development of practical performance models which inform the operation of these systems, as well as identifying possible strategies for improving the NSR catalyst materials. This chapter describes the results of diffraction, microscopy and spectroscopy studies that have identified monolayer and three-dimensional BaO and Ba(NO₃)₂ phases that undergo significant changes in morphology during NO_x uptake and release. Similarly, other gas-phase species present at considerable quantities in the exhaust, notably H₂O, CO₂ and oxides of sulfur, can react with the NO_x storage material and, as such, significantly affect its NO_x uptake and release processes. These reactions and their effects have also been the subject of numerous studies as also described in this chapter.